In some vehicles, such as hybrid vehicles, a high level system controller may control a plurality of power and/or torque sources for propelling the vehicle. For example, the controller may allocate a total drive request among the various sources in different ways depending on operating conditions. During the allocation, a vehicle controller may consider one or more factors such as the available output range of the individual torque sources. For example, an engine may have a variable maximum torque output depending on operating conditions, and it may further change as the engine ages, etc.
One example approach for dynamic torque allocation is shown by Martin et al. in U.S. Pat. No. 7,967,720. Therein, the maximum torque allowed for the engine is adjusted during vehicle operation based on vacuum demand for various conditions, such as for fuel vapor purging. Specifically, when there is insufficient vacuum for purging a fuel canister, the maximum engine torque is limited to a level that provides the required purge vacuum. The remaining operator torque demand is then allocated to the hybrid vehicle electric motor.
However the inventors herein have identified a potential issue with such an approach. The adjusting of maximum engine torque to meet vacuum demand can result in too much limiting of the maximum torque, which hurts fuel economy. As such, it may be difficult to trade-off best fuel economy and maximum engine performance. The setting for best performance will have a maximum engine torque in the borderline spark reduction region, which is inefficient and degrades fuel economy. On the other hand, operating at a lower maximum engine torque that is at MBT spark under all conditions sacrifices engine performance and is not optimally efficient when not operating at worst case conditions. Additional variability in fuel octane rating, and engine internal conditions (such as in-cylinder temperature, compression, deposits, knock detection, etc.) make it more difficult to accurately form an open-loop prediction of the highest maximum torque available when spark is at or near MBT.
In one example, the above issue may be at least partly addressed by a method for a hybrid electric vehicle comprising: dynamically limiting a maximum available torque for an engine based on each of an operator selected vehicle performance mode and a change in torque demand, the maximum available torque limited between a first torque limit based on performance and a second torque limit based on fuel economy; and providing motor torque to the vehicle based on the dynamic limiting. In this way, a better balance can be struck between engine performance and fuel economy.
For example, in response to an operator pressing an “ECO” button of a vehicle, a fuel economy mode of vehicle operation may be selected. Accordingly, the settings of various engine operating parameters may be adjusted so as to improve fuel economy while minimizing reduction in performance when fuel economy is the priority. In particular, a maximum available engine torque may be limited. The constrained maximum available engine torque may be learned (e.g., learned up or learned down) during engine operation, at different engine speed ranges, based on a spark retard torque ratio (that is, a ratio of maximum engine torque at borderline spark relative to maximum torque at MBT). The learned maximum engine torque may allow the most torque to be extracted from the engine without entering an inefficient state. The constrained maximum engine torque may be applied when operating in the economy mode both during steady-state as well as transient conditions. In comparison, when the “ECO” button is not actuated, and a normal mode is selected where vehicle performance is of priority, the constrained maximum engine torque may be applied only when operating in steady-state conditions. During transient conditions, an unconstrained maximum engine torque may be used, at the cost of a small fuel penalty, to allow vehicle accelerations to be improved.
In this way, changing environmental and engine conditions are better accounted for and the best available engine torque is available for vehicle performance while still delivering the best fuel economy. By dynamically the degree of limiting of the maximum engine torque, and the allocation of a drive torque between an engine torque and a motor torque, a more aggressive use of the available engine is possible. In addition, a smaller, less costly battery can be used to achieve equivalent vehicle performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.